With the current popularity of portable communication devices and developed semiconductor fabrication technology, high speed and high performance transistors are more densely integrated on semiconductor dies. Consequently, the amount of heat generated by the semiconductor dies increases significantly due to the large number of transistors integrated on the semiconductor dies, the large amount of power passing through the transistors, and the high operation speed of the transistors. Accordingly, it is desirable to package the semiconductor dies in a configuration for better heat dissipation.
A conventional approach to enhance thermal performance of a package is to place a heat spreader directly over the semiconductor dies. However, the height variation of different dies makes it very difficult to directly connect the heat spreader with each semiconductor die. A compliant material might be applied to each die to eliminate the height variation. As such the heat spreader may make contact with each die. However, the compliant material used at the interface of each die typically has a relatively high thermal resistance, and will significantly increase the thermal resistance of the whole package. Furthermore, due to different amounts of the compliant material applied to each die (for even height of each die), the increase of thermal resistance for each die will be different.
Accordingly, there remains a need for improved semiconductor package designs to accommodate the increased heat generation of high performance dies and to avoid using the compliant material between the interface of each die and the heat spreader.